As for recent multilayer printed circuit boards, downsizing of the via holes has been rapidly promoted in parallel with the circuit miniaturization required to multilayer printed circuit boards. In this connection, the conventional mechanical drilling machining has become difficult to be applied to such microfabrication, and alternatively the laser drilling machining has been generally prevailing in such microfabrication.
As the laser drilling machining has come into wide use, the multilayer printed circuit boards manufactured by use of the conventional glass-epoxy base material of FR-4 prepreg have proved to be poor in laser drilling workability. It is the glass cloth incorporated as skeletal component in glass-epoxy base material that has been first recognized as problematic. Glass cloth is a woven stuff and glass itself is poor in laser machining workability, and hence drilling with satisfactory precision has been impossible with glass cloth.
Thus, the present inventors have been supplying to the market the copper foil with resin in which exclusively a semi-cured resin layer without incorporating a skeletal component is provided on the surface of the copper foil. Accordingly, it has been made possible to manufacture, with applying the built-up processing method but without using prepreg, the copper clad laminates which are excellent in the laser drilling workability, this situation having made it possible to supply high quality multilayer printed circuit boards. In other words, the copper foil with resin has the characteristics that it is light in weight and excellent in the laser drilling workability owing to the absence of the skeletal component therein, and simultaneously, the copper foil with resin has the following drawbacks owing to the absence of the skeletal component therein.
Namely, there has been a problem that a copper clad laminate manufactured only with the copper foil with resin is insufficient in the mechanical strength of the resin layer thereof against such external forces as bending, tensile, and impact forces. The copper foil with resin has no reinforcing material, and thus the quality control is difficult for a copper clad laminate manufactured only with the copper foil with resin, since the thickness of an insulating layer within a laminate layer varies extremely widely in a system having nonuniform copper circuit densities in the inner layer circuits. The copper foil with resin is a material large in thermal expansion coefficient, and thus tends to generate a stress in the interface with a different type of material, for example, with a copper circuit, resulting in adverse effects to the board reliability. Among other drawbacks pointed out, there is a drawback that a copper clad laminate manufactured only with the copper foil with resin is low in strength so that pads sink into the laminate during wire bonding of IC chips, resulting in failure in obtaining stable bonding.
On the other hand, in the field of prepreg, there have also been supplied products in which the skeletal component is devised so that the laser drilling workability is improved while retaining the above described mechanical strength. To be more specific, it has been said that when glass cloth is used as the skeletal component, the laser drilling workability is generally degraded; accordingly, it has become general to use the nonwoven glass fabric as the skeletal component instead of the woven glass cloth. The use of nonwoven fabric has improved the nonuniformity in cloth thread as seen when the nonwoven fabric is used as the skeletal component, thereby significantly improving the laser drilling workability.
However, as for the prepreg containing a skeletal component, there is usually adopted a method in which the skeletal component is impregnated with a resin component, and then dried, which method accordingly causes problems.
To be more specific, the nonwoven fabric itself is inferior in strength to the woven glass cloth, and accordingly, there has been a drawback that when the impregnated unwoven fabric is taken out of the impregnating resin, sometimes the impregnated nonwoven fabric tends to break adversely owing to the weight of the resin impregnating thereinto. Even with the woven glass cloth, a similar drawback has tended to occur with decreasing cloth thickness. Thus, although it is desirable to use either a thinner nonwoven fabric or a thinner woven cloth, actually there has been a restriction in reducing the thickness of a nonwoven fabric or a woven cloth, in consideration of the strength required to the insulating layers incorporated into a fabricated copper clad laminate.
Thus, there has been attempted to supply those prepregs which use either thinner nonwoven fabric or woven cloth, through achieving the object that the insulating resin layer is reduced in thickness for the purpose of weight reduction, and simultaneously reducing the resin content impregnated into the nonwoven fabric or the woven cloth. A copper clad laminate is manufactured by press working to make a copper foil adhere onto the surface of a prepreg. In this case, the copper foil has been before hand subjected to the nodular treatment to form concavities and convexities on the surface thereof, so that the nodular treatment surface of the copper foil go into the resin portion in the base material to increase the adhesion strength through obtaining the anchoring effect; when the impregnated resin amount is made to be equal to or lower than a certain level, the skeletal component and the nodular treatment surface of the copper foil come into contact with each other, resulting in degraded adhesion of the base material resin and thereby resulting in the degraded peel strength of the laminate; and in addition, it has become anticipated that the direct contact between the skeletal component and the copper foil possibly facilitates the migration of the copper foil along the skeletal component fibers.
From the above, there have been desired a material and a method which material and method make it possible to prevent more securely the contact between the nodular treatment surface of the attached copper foil and the skeletal component, through raising the resin content ratio in an insulating resin layer by making the skeletal component incorporated into the insulating resin layer of a fabricated copper clad laminate as thin as possible.